1. Field of the Invention
The present invention relates generally to inductive devices and methods for fabricating such devices, and more particularly to a three-dimensional coil inductor.
2. Discussion of Background Art
Inductors are a fundamental electromagnetic component necessary to a wide variety of devices, such as actuators, relays, motors, DC-to-DC converters and RF circuits. Inductors having large inductances typically consist of wires wrapped around a bulk dielectric or ferromagnetic core, such as those used in power converters and relays. Power delivery and conversion subsystems incorporating such inductors are often one of the largest, heaviest, and most physically awkward components of an electronic system. Relays also require large inductances for proper operation and, as a result, are typically very bulky and complex devices. In addition, relays currently are built with a large number of discrete parts, which are often laboriously fabricated.
Small inductors are substantially two-dimensional (i.e. their lateral dimensions greatly exceed their vertical dimension) thin-film devices etched on either circuit boards or silicon wafers. Spiral inductors fabricated on a silicon substrate are one such example. Such inductors typically have very small inductances and a limited usefulness due to a magnetic coupling with the silicon substrate resulting in eddy-current loss and a higher than normal parasitic capacitance. As a result, the inductance, energy storage and power handling of such inductors are very limited. Likewise, miniature electrostatic relays have been produced, using thin film techniques, however, they often fail to develop sufficient magnetic force to ensure a reliable metal-to-metal contact.
Currently, there exists a need in the art for physically small inductors having relatively large inductances. For instance, RF communication devices are becoming increasingly popular, and competition is driving the development of smaller and more efficient RF devices. A typical RF communication device, such as a cellular telephone operates at around 1 GHz and requires inductors with an inductance on the order of 5nH, a Q of at least 10, and a self-resonant frequency well in excess of the operating frequency.
Many of these inductors are required to fabricate oscillators, filters and matching networks that go into such devices; however, current manufacturing techniques fail to reach such performance levels. For example, conventional aluminum spiral inductors fabricated on standard silicon substrates achieve Q-factors of around only 3 at 1 GHz.
In addition, smaller power converters and relays for a variety of applications are also needed which can not be manufactured using current techniques.
In response to the concerns discussed above, what is needed is an inductor that overcomes the problems of the prior art.
The present invention is a three-dimensional coil inductor and a method for fabricating said inductor. The inductor can take two forms. A first inductor includes a substrate, such as a silicon wafer; a set of lower electrically conductive traces positioned on the substrate; a core placed over the lower traces; a set of side electrically conductive traces laid on the core and the lower traces; and a set of upper electrically conductive traces attached to the side traces so as to form the first inductor. A second inductor includes a substrate including a recess; a set of lower traces placed on the substrate within the recess; a set of side traces placed on the substrate and attached to the set of lower traces within the recess; a core positioned over the lower traces; and a set of upper traces overlaying the core and attached to the set of side traces so as to form the second inductor.
Fabrication of the first inductor includes the steps of forming a set of lower traces on a substrate; positioning a core over the lower traces; forming a set of side traces on the core; connecting the side traces to the lower traces; forming a set of upper traces on the core; and connecting the upper traces to the side traces so as to form a coil structure. Fabrication of the second inductor includes the steps of providing a substrate having a recess; forming a set of lower traces within the recess; forming a set of side traces within the recess; connecting the side traces to the lower traces; positioning a core over the lower traces; forming a set of upper traces on the core; and connecting the upper traces to the side traces so as to form a coil structure.
The present invention permits fabrication of miniature three-dimensional versions of efficient, macroscopic coil or solenoidal inductors as well as other electromagnetic components and systems. The present invention is particularly useful in RF applications, such as cellular phones, where improvements in IC technology can yield a significant competitive advantage. The three-dimensional inductor structure minimizes capacitive coupling to the substrate and eddy current loss. Low fabrication temperatures enable the inductor to be placed on top of a substrate having active devices without affecting characteristics of those devices. Thick copper traces can be used to reduce series resistance, ensuring a high Q factor for RF applications, possibly enabling construction of monolithic wireless transceivers. Monolithic indicates that inductor windings are deposited on a three dimensional inductor core, in contrast to current practices of either wrapping a wire around a three dimensional core, or depositing metal on a substantially two dimensional core.
Electromagnets which can handle higher coil currents and operate relays can also be fabricated using the techniques of the present invention. Compact and integrated power converters can also be fabricated. Such power converters could find application in various power generation, storage and distribution systems. Performance of such devices would far exceed that of conventional component-based systems in terms of power density, efficiency, form factor and cost.
These and other aspects of the invention will be recognized by those skilled in the art upon review of the detailed description, drawings, and claims set forth below.